Simulation 2: Nominal and True Stresses and Strains An engineering stress-strain curve is constructed from the load elongation measurements (Fig. Neglecting this has only a small effect on the appearance of most stress-strain curves. In the tension test a specimen is subjected to a continually increasing uniaxial tensile force while simultaneous observations are made of the elongation of the specimen. Since a typical Young's modulus of a metal is of the order of 100 GPa, and a typical yield stress of the order of 100 MPa, the elastic strain at yielding is of the order of 0.001 (0.1%). Note that the elastic strains are not shown on this plot, so nothing happens until the applied stress reaches the yield stress. Assume that a material has a true stresstrue strain curve given by, calculate the true ultimate tensile strength and the engineering UTS of this material. After plotting the stress and its corresponding strain on the graph, we get a curve, and this curve is called stress strain curve or stress strain diagram. In this, the stress is plotted on the y-axis and its corresponding strain on the x-axis. The graph on the right then shows true stress-true strain plots, and nominal stress-nominal strain plots, while the schematic on the left shows the changing shape of the sample (viewed from one side). Stress strain curve is the plot of stress and strain of a material or metal on the graph. The applied force, F, is then progressively raised via the third slider. The sliders on the left are first set to selected σ Y and K values. True Strain (calculated from Engineering stress/strain data): Material Data: Al 6061 Y 40 ksi TS 49 ksi True Stress vs. Where σ Y is the yield stress and K is the work hardening coefficient. For an applied force F and a current sectional area A, conserving volume, the true stress can be written Conversely, under compressive loading, the true stress is less than the nominal stress.Ĭonsider a sample of initial length L 0, with an initial sectional area A 0.
After a finite (plastic) strain, under tensile loading, this area is less than the original area, as a result of the lateral contraction needed to conserve volume, so that the true stress is greater than the nominal stress. The true stress acting on the material is the force divided by the current sectional area. Note that the true stress always rises in the plastic, whereas the engineering stress rises and then falls after going through a maximum. The true stress is the load borne by the sample divided by a variable the instantaneous area. In fact, these are “ engineering” or “ nominal” values. The engineering stress is the load borne by the sample divided by a constant, the original area. It is common during uniaxial (tensile or compressive) testing to equate the stress to the force divided by the original sectional area and the strain to the change in length (along the loading direction) divided by the original length. Department of Materials Science and Metallurgy at University of Cambridge.Dissemination of IT for the Promotion of Materials Science (DoITPoMS).